A need for higher resolution image production is well documented in the art. A close inspection of letters and graphic images on a computer screen or a hard copy print out reveal jagged lines causing an unsightly rough appearance of displayed and printed images.
One method of ameliorating this problem, disclosed in U.S. Pat. No. 4,437,122 to the Xerox Corporation involves isolating a center pixel, matching the 3 by 3 pixel pattern surrounding and including the center pixel with a table of "standard pixel formats", and then subdividing the center pixel area into enhanced "subpixels".
Another method utilizes pattern matching templates operating on a window with a central bit. See U.S. Pat. No. 4,847,641 assigned to the Hewlett Packard Corporation.
One disadvantage in the prior art is the use of "standard pixel formats" and templates. When the selected pixel and the surrounding pixels to be analyzed are few in number, for example a 3 by 3 pixel matrix, and when the pixels are only bi-valued (black or white), the total number of possible input patterns is small. A 3 by 3 matrix of input pixels, each either black or white, has a total of only 512 possible patterns which, in contemporary hardware or software can easily be handled by placing 512 hand selected "answers" ("templates" or "standard pixel formats") into a look-up table. The analysis of only a 3 by 3 pixel matrix is very error prone, however, since the pattern analyzed can be indicative of a line angled at 45.degree. or the beginning of a curve. Analyzing a larger subset of the input pixel matrix requires the formation of a very large unwieldy number of standard pixel formats or templates.
The templates disclosed in the Hewlett Packard patent are similar to input windows except for having a third pixel state in addition to black or white. The third state is called a "don't care" condition. This allows each template to represent many possible inputs, specifically two to the power of the number of "don't care" conditions in a given template, thus greatly reducing the number of templates required. Such tri-valued templates, however, are still very limited. For example, if one tiny feature is to be allowed two different ways, two completely separate templates are required.
Although it may be fairly straight forward to straighten out a jagged 45.degree. line according to this prior methodology, it is nearly impossible to use standard pixel formats or templates to account for the numerous possible permutations of angled lines, curves, and other features inherent in the display of alpha numeric characters and graphical displays. In addition, the prior art methodologies are generally constrained to bi-valued input bitmaps.
Another type of system involves the use of "anti-aliasing" filters which are linear. Anti-aliasing filters do not make inferences based on real world properties of edges such as slope continuity. Anti-aliasing filters remove spurious a high frequency information which is an artifact of the quantization process but they do not replace the spurious high frequency information with inferred hypothetical high frequency information.